This invention relates generally to extractive distillation processes and extractive distillation compositions for enhancing the separation of hydrocarbon compounds via extractive distillation. The extractive distillation compositions comprise both organic sulfoxides and organic sulfones.
It is difficult to efficiently and economically separate mixtures of organic compounds having similar chemical characteristics, and nearly the same boiling point and/or volatility. Conventional fractional distillation can be expensive and inefficient in this situation, in that large and expensive columns are required, which have large numbers of plates, and utilize high reflux ratios, with correspondingly high energy consumption rates.
Extractive distillation is a technique for separating certain close boiling mixtures. In extractive distillation, a high boiling extractive distillation solvent is typically introduced into a distillation zone containing feed that is to be separated. The high boiling solvent interacts with the feed mixture (which is typically lower boiling than the extractive distillation solvent) to effectively decrease the volatility of some feed mixture components, typically the more polar feed components, so that the less polar feed components can be separated by distillation overhead. The more polar feed components become relatively concentrated in the extractive distillation solvent/composition, and typically exit the distillation zone as a heavy fraction.
An extractive distillation process has been described in the article entitled xe2x80x9cExtractive Distillation Saves Moneyxe2x80x9d by Ian Sucksmith, Chemical Engineering, Jun. 18, 1982, pages 91-95. Other literature sources relating to conventional extractive distillation techniques include the xe2x80x9cHandbook of Separation Techniques for Chemical Engineersxe2x80x9d by Phillip A Schweitzer, McGraw-Hill Book company, 1984, pages 13-53. A variety of methods for employing extractive distillation to separate various classes of hydrocarbons, such as aromatics, olefins, or cycloalkanes, from other close-boiling hydrocarbons, such as paraffins, are known in the hydrocarbons industry. In particular, many refinery streams comprise xe2x80x9cBTXxe2x80x9d streams composed of close boiling mixtures of aliphatic hydrocarbons (such as isomers of heptane and octane) and aromatic hydrocarbons (such as benzene, toluene, and xylenes).
A variety of extractive distillation compositions have been reported, and a select few are practiced commercially. The most common of these commercially practiced extractive distillation compositions include polyethylene glycols and mixtures thereof (i.e. xe2x80x9cUdexxe2x80x9d processes). U.S. Pat. Nos. 3,714,033 and 4,921,581 disclose the use of polyalkylene glycol solvents toward this end. Triethylene glycol and tetraethylene glycol are examples of solvents that are commonly employed in such xe2x80x9cUdexxe2x80x9d processes.
Other processes for extractive distillation of hydrocarbons have employed organic sulfones as extractive distillation solvents. U.S. Pat. Nos. 2,033,942 and 2,831,039 described the use of dialkyl sulfones, including dimethyl sulfone, in such separations. U.S. Pat. No. 4,401,517 relates to the use of C4-C8 sulfones as selective extractive distillation solvent. U.S. Pat. No. 3,146, 190 described the use of sulfolane (tetramethylene sulfone) as a selective extraction solvent for the purification of pyrolysis fuels and catalytically reformed gasolines. U.S. Pat. Nos. 3,466,346 and 3,723,256 describe refinements of sulfolane-based extractive distillation processes. Sulfolane is also commercially employed as an extractive distillation solvent for the extractive distillation of hydrocarbon compounds.
Dimethyl sulfoxide (DMSO) has also been described as a solvent in extractive distillation applications. Early examples of DMSO based aromatic extraction processes are described in French Patent No. 1,407,134 and by Krause (Chem. Eng. 54-56 (1966)). British Patent No. I373,325 to Rhone-Progil described the use of DMSO to separate mixtures of meta- and para-dichlorobenzene by extractive distillation. U.S. Pat. No. 5,399,244 to Gentry, et al., described the use of aqueous solutions of dimethyl sulfoxide (DMSO) in separating benzene and other aromatic hydrocarbons from mixed aromatic and non-aromatic hydrocarbon streams.
In the commercially practiced embodiments of extractive distillation processes, the solvent (typically triethylene glycol, tetraethylene glycol, sulfolane, and/or DMSO) is believed to be employed in relatively pure form. Nevertheless, there have been reports the use of additives to enhance extractive distillation performance. Water-sulfolane mixtures are disclosed in U.S. Pat. No. 5,849,982. U.S. Pat. No. 5,032,232 describes combinations of N-alkyl-2-thiopyrrolidones and sulfolane for this purpose. U.S. Pat. No. 4,024,028 describes the use of mixtures of dimethyl sulfone, methyl ethyl ketone and sulfolane for the extractive distillation of hydrocarbon mixtures. One study examined the selectivity and solvent power of various DMSO/ethylene glycol mixtures (O. P. Dimitieva et. al., Obshch. Prikl. Chim. 6, 119-122 (1974)). The separation of toluene from close-boiling non-aromatics by extractive distillation with phthalic anhydride in combination with a variety of solvents, including dimethyl sulfoxide, was described in U.S. Pat. No. 4,363,704. Other additives for combination with various extractive distillation solvents have been reported, but many of them are expensive, or have insufficient thermal and/or chemical stability for practical use in extractive distillation compositions.
U.S. Pat. No. 2,831,039 to Nevitt described the use of various short chain (2-4 Carbon) dialkyl sulfones, including dimethyl sulfone, simultaneously with water from about 1 to about 15% by weight, for separating aromatic hydrocarbons from a non-viscous neutral oil containing aromatic and aliphatic hydrocarbons boiling in the gasoline boiling range. Nevitt also disclosed the use of a wide variety of optional diluents, in a concentration range from about 0.1 to about 20% by volume, based on the volume of sulfone solvent employed. Among the many diluents disclosed was dimethylsulfoxide. The above described patents are incorporated hereinbelow by reference in their entirety for their disclosures of extractive distillation compositions and processes.
Polyalkylene glycols or sulfolane are the most widely commercially used extractive distillation solvents used in modern hydrocarbon processing. The glycol systems suffer from relatively low extraction selectivity and aromatic loading capabilities as compared to sulfolane. Polyalkylene glycols also suffer from significant thermal and/or oxidative degradation on an annual basis, and therefore necessitate regular makeup of their volume, which incurs significant expense. Use of sulfolane results in improved extraction selectivity and loading levels, as compared to polyalkylene glycols. Nevertheless, the loading capacity of sulfolane is markedly inferior to that of DMSO, at comparable solvent to hydrocarbon feed ratios. While the use of DMSO results in high capacity, the selectivity of the extractive distillation separation for separation of aromatics from aliphatics is poor as compared to the use of either polyethylene glycols or sulfolane.
It has been estimated that approximately 110 glycol based extractive distillation units and about 75 sulfolane based extractive distillation units are currently being operating commercially worldwide. It is also believed that a handful of DMSO based units operate commercially, primarily in France. Wu et al. estimate (Chemical Engineering, page 139, March 1998) that for a typical industrial BTX extraction unit, a 1% increase in aromatics recovery would result in savings of up to $100,000 per year. Such savings derive from both decreased need for capital investment in equipment, and decreased unit energy requirements (electricity, steam, etc.) needed to perform the extractive distillation.
Thus, both technical and financial considerations provide a continuing need for developing novel and improved extractive distillation compositions. In particular, there is a continuing need for improved extractive distillation processes which could provide a combination of improved selectivity and capacity/loading, as compared to known solvents or compositions and processes.
The instant invention relates to improved extractive distillation compositions comprising both organic sulfoxides and organic sulfones. The invention also relates to extractive distillation processes that employ the extractive distillation compositions of the invention to at least partially separate mixtures of hydrocarbon compounds. The extractive distillation compositions and/or processes may provide surprising improvements in the separation of certain classes of hydrocarbon compounds, as compared to prior art extractive distillation solvents and/or compositions. The improved separation of the hydrocarbon compounds may be manifested as an unexpectedly large improvement in the selectivity of the separation of the hydrocarbon compounds. The improved separation of the hydrocarbon compounds may also be manifested as an unexpectedly large improvement in the capacity of the extractive distillation composition, i.e. an increase in the loading of certain hydrocarbon compounds in the extractive distillation composition after other hydrocarbon compounds have been removed by extractive distillation. In some embodiments of the invention, use of the extractive distillation compositions of the invention in an extractive distillation process simultaneously and even more unexpectedly provides improvements in both loading and selectivity for the separation of the hydrocarbons, as compared to prior art extractive distillation compositions and processes.
Therefore, in one aspect, the invention relates to an extractive distillation composition for at least partially separating a mixture of hydrocarbon compounds comprising:
a. at least on one organic sulfoxide of the formula
R1xe2x80x94SOxe2x80x94R2, and
b. at least on one organic sulfone of the formula
R3xe2x80x94SO2xe2x80x94R4
xe2x80x83wherein R1, R2, R3, and R4 have at least one carbon atom and can be the same or different.
In certain embodiments, the extractive distillation compositions of the invention comprise
a. dimethyl sulfone, and
b. dimethyl sulfoxide.
In another aspect, the invention relates to a process for at least partially separating mixtures of aromatic and non-aromatic hydrocarbon compounds by extractive distillation that employs an extractive distillation composition comprising:
a. at least on one organic sulfoxide of the formula
R1xe2x80x94SOxe2x80x94R2, and
b. at least on one organic sulfone of the formula
R3xe2x80x94SO2xe2x80x94R4
xe2x80x83wherein R1, R2, R3, and R4 have at least one carbon atom and can be the same or different, and
wherein the at least one organic sulfoxide is present in an amount effective to provide improved separation of the aromatic and non-aromatic hydrocarbon compounds, as compared to a process employing the same extractive distillation composition not comprising the organic sulfoxide.
In yet another aspect, the invention relates to a process for separating hydrocarbon compounds of similar volatility by extractive distillation, comprising the steps of:
a. contacting a feed mixture with an extractive distillation composition, wherein the feed mixture comprises at least
i. at least one first hydrocarbon compound, and
ii. at least one second hydrocarbon compound; and
b. distilling the feed mixture within an extractive distillation zone to at least partially separate the feed mixture into a first stream enriched in the first hydrocarbon compound, and a second stream enriched in the second hydrocarbon compound;
wherein the extractive distillation composition comprises
i. dimethyl sulfone, and
ii. greater than about 20 percent by volume of dimethyl sulfoxide.
In yet another aspect, the invention relates to process for at least partially separating hydrocarbon compounds by extractive distillation that employs an extractive distillation composition comprising:
a. greater than about 21% by volume of dimethylsulfoxide, and
b. greater than about 21% by volume of dimethylsulfone.
In one embodiment, the invention provides a process for separating hydrocarbon compounds of similar boiling points by extractive distillation, comprising the steps of:
a. contacting a feed mixture with an extractive distillation composition within an extractive distillation zone, wherein the feed mixture comprises at least
i. a first hydrocarbon compound, and
ii. a second hydrocarbon compound; and
b. distilling the feed mixture to at least partially separate the feed mixture into a first stream enriched in the first hydrocarbon compound, and a second stream enriched in the second hydrocarbon compound; and
wherein the extractive distillation composition comprises
i. at least about 21% by weight of an organic sulfoxide of the formula
R1xe2x80x94SOxe2x80x94R2, and
ii. at least about 21% by weight of an organic sulfone of the formula
R3xe2x80x94SO2xe2x80x94R4
xe2x80x83wherein R1, R2, R3, and R4 have at least one carbon atom and can be the same or different.
In yet a different aspect, the invention relates to a process for separating hydrocarbon compounds of similar boiling points by extractive distillation, comprising the steps of:
a. introducing a feed mixture into an extractive distillation zone having an upper portion and a lower portion, wherein the feed mixture comprises at least
i. a first hydrocarbon compound comprising a C7-C10 aliphatic compound, and
ii. a second hydrocarbon compound comprising benzene, toluene, ortho-xylene, meta-xylene, para-xylene, or a mixture thereof; and
b. introducing an extractive distillation composition into the upper portion of the extractive distillation zone; and
c. distilling the feed mixture to at least partially separate the feed mixture into a first stream rich in the first hydrocarbon compound, and a second stream rich in the second hydrocarbon compound;
wherein the extractive distillation composition comprises:
i. from about 25 to about 75 parts by weight of dimethylsulfoxide,
ii. from about 25 to about 75 parts by weight of dimethylsulfone, and
iii. from about 0.1 to about 15 parts by weight of water.
Additional advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.